1. Field of the Invention
The invention relates to a receiver comprising an RF section, a mixer stage and a signal processing section, said mixer stage receiving a mixing frequency from a frequency synthesis circuit which comprises a phase-locked loop provided with a phase detection device having a current output, a loop filter comprising a .pi.-shaped RC network having an input shunt path and an output shunt path which are connected to respective ends of a series resistor incorporated in a series path, and to ground and comprise first and second capacitances, respectively, and with a voltage-controlled oscillator, a reference frequency being applied to said phase detection device.
2. Description of the Related Art
A phase-locked loop for use in a frequency synthesis circuit of such a receiver is known per se, for example from U.S. Pat. No. 3,316,497.
In the known phase-locked loop, the controlled oscillator is coupled to the phase detection device via a frequency divider. The reference frequency is also applied to the phase detection device from, for example, a crystal oscillator. In the phase-locked state of the loop, phase difference between the frequency-divided oscillator signal, on the one hand, and the reference frequency, on the other hand, are converted in the phase detection device into a phase difference signal which is negatively fed back in the loop from the output to the input of the phase detection device and is thereby suppressed. As a result, an accurate phase synchronization of the frequency-divided oscillator signal with the reference frequency takes place in this phase-locked state and the oscillator frequency is a factor determined by the dividend or division factor of the frequency divider multiplied by the reference frequency. By varying said dividend, the oscillator frequency, which is applied as a mixing frequency to the mixer stage, can be adjusted to a desired tuning frequency.
For a correct operation of the phase-locked loop, the loop parameters should satisfy conflicting requirements: for example, the loop selectivity is to be chosen as large as possible for a maximal phase synchronization. However, a large loop selectivity is accompanied by a large phase shift which jeopardizes the loop stability. The impedance levels should also be noiselessly adjusted to correct values so as to prevent the loop circuits from influencing each other.
In the known phase-locked loop, use is made of active filter sections for this purpose, which, together with said .pi.-shaped RC network, form part of a loop filter. In these active filter sections, capacitances, series resistors and operational amplifiers ar used for amplifying and selecting a dc control signal for the voltage-controlled oscillator. Particularly because of the series resistors and the amplifiers, the use of such active filter sections in a phase-locked loop introduces a number of unwanted effects in the loop, such as:
the occurrence of unwanted sideband signals around the oscillator frequency at intervals with the order of magnitude of the reference frequency, caused by leakage currents in the amplifiers; PA0 an unwanted frequency modulation of the oscillator signal as a result of insufficient suppression of power supply voltage variations or ripple in the amplifiers; PA0 noise which becomes manifest as an unwanted sideband noise in the oscillator signal.